Apologies for chipping in, but when you wrote about this family in April, I made a suggestion about the third son in the records, who also must have been left behind in the IOM.
On the Down County Museum Prisoners Database, they recorded Sarah's children as 3 male, 2 female. You wrote that she took 2 girls and 1 boy with her to Australia.
I thought that the following could have been her other son? As she was transported in 1835, he could have turned 8 and therefore been left behind. If he was the other son, his history in registers and censuses etc. could help locate Andrew.
James LYNCH Head M Male 54 Ireland Fireman
Mary A.S. LYNCH Wife M Female 53 Douglas, Fireman's Wife
Alexander LYNCH Son U Male 23 Douglas
Elizabeth E. LYNCH Daur U Female 18 Douglas, Dressmaker
Dwelling 13 Queen St
Census Place Onchan, Isle of Man, Isle of Man
Family History Library Film 1342346
Public Records Office Reference RG11
Piece / Folio 5603 / 66
Page Number 24
Married Mary Ann Finigan in 1848?